There are hundreds of thousands of various storage tanks in the United States that are used to contain liquids. Storage tanks may be used to store, for example, water, waste, and consumer products such as milk. Storage tanks may also be placed in several locations such as on a vehicle or above or below the ground surface. In the oil and gas industry, storage tanks are used to store hydrocarbons at various locations, including oil refineries and smaller, remote tank batteries at oil production sites. Storage tanks for upstream oil and gas production are typically kept at or near atmospheric pressure for safety reasons and to allow the hydrocarbon liquid to be loaded into trucks for transportation to markets.
One issue with these storage tanks is the natural production of vapors. Light hydrocarbons in the crude oil or condensate—including methane and other volatile organic compounds, natural gas liquids, hazardous air pollutants, and some inert gases—vaporize or “flash” and collect in the space between the surface of the crude oil and the roof of the storage tank. This flashing of hydrocarbons reduces the volume of the crude oil stored in the tank. These vapors are often ultimately released to the atmosphere through a pressure safety valve or other vent mechanism resulting in increased air pollution and sometimes putting those facilities in violation of local, state, and federal air pollution regulations. There are several approaches to handle and control these vapors. One approach is to vent the vapors from the storage tanks and into the atmosphere. This releases pollutants into the atmosphere. When relevant governmental regulations are imposed, another approach is to ignite and flare the vapors so that the vapors are combusted. Thus, in the case of increasing governmental regulations, an operator may have to install substantial control devices to mitigate the vapor emissions. In either approach, the venting or burning of vapors represents wasted resources, atmospheric pollution, and reduced oil production.
One way to mitigate the accumulation of vapors inside of a storage tank is to incorporate a movable or floating roof into the design of the storage tank. Some prior art storage tanks in refineries have a cylindrical outer wall with a circular roof that rises and falls within the cylindrical outer wall to match the liquid level in the storage tank. This reduces the loss of vapors from the crude oil and petroleum products. However, these designs are economical only for large storage tanks. U.S. Pat. No. 8,061,552, which is incorporated herein in its entirety by reference, notes that floating-roof storage tanks are commonly between 15 and 400 feet in diameter and hold up to 1.5 million barrels of liquid or more. As such, the floating roofs are incorporated into the design of the storage tank and must be installed when the storage tank is constructed.
In contrast, floating roofs are not economical for smaller storage tanks, particularly for smaller storage tanks that have already been built and would require retrofitting. Normally, these storage tanks hold considerably less than 50,000 barrels of liquid and more frequently less than 5,000 barrels. One example of these smaller storage tanks is a tank that stores oil from stripper wells. These smaller storage tanks have fixed roofs and typically have a small access port to allow inspection and measuring. These smaller tanks handle vapor formation using the atmospheric vent approach, and it is uneconomical to retrofit the roof of these smaller storage tanks with a floating roof. In addition, environmental regulations that require burner units or other control devices to be installed may also render the producing oil well uneconomical.
Therefore, there is a need for a device, a system, and/or a method for controlling the vapor flash of hydrocarbon liquids in smaller storage tanks to prevent the loss of crude oil and petroleum products by flashing, and to prevent polluting gases from being released into the atmosphere.